Increasing miniaturization of electronic circuitry has required improved techniques in thin film technology involving the deposition and removal of molecular layers of materials onto and from a workpiece such as a silicon wafer, which conventionally serves as a substrate for Very Large Scale Integrated (VLSI) circuits. One well known plasma processing technique for removing thin layers of material is reactive ion etching or plasma etching. In this process, an electrical gas discharge, or plasma, is created by imposing a direct current (dc) voltage or, preferably, a radio frequency (rf) voltage between electrodes in a space occupied by a normally non-reactive gas at low pressure. Energetic electrons emitted from the negative electrode collide with neutral gas atoms or molecules to create ions or other reactive species and additional electrons, thereby initiating and maintaining a highly conductive glow discharge in a region adjacent to the electrode. This glow discharge or plasma is separated from the electrode surface by a dark space or plasma sheath.
Since the plasma is essentially equipotential, the voltage drop between the plasma and the electrode occurs in the plasma sheath, and the direction of the electric field is normal to the electrode surface. Consequently, the ions and other reactive species generated in the plasma, which typically carry a positive charge, are attracted to the surface of an oppositely charged electrode surface and travel from the plasma to the surface primarily in a direction parallel to the electric field lines. In the plasma etching process, the above-mentioned electrode serves as a substrate support, so when the ions or reactive species reach the surface of the substrate they either activate or take part in a chemical reaction resulting in the desired removal of material from the substrate surface.
U.S. Pat. No. 4,422,896 of W. H. Class et al. discloses the use of magnetic enhancement for plasma etching process. In one proposed arrangement, an electrode is formed with a prismatic body having several flat faces, constituting substrate support surfaces, arranged symmetrically about an axis. First and second magnetic pole pieces of opposite polarity project outwardly from the faces and extend completely around the electrode body at respective ends of the body, the resulting structure being basically spool-shaped. A magnetic field extending between the pole pieces thus forms a continuous belt around the body of the electrode adjacent to the substrate support surfaces.
The symmetrical prismatic spool shape of this previously proposed electrode provides multiple substrate support surfaces and is particularly suited to be mounted for rotation about its axis so that, in bias sputtering applications, each face can be directed in succession toward one or more sputtering targets. The prismatic shape also permits loading or processing a large number of substrates for a given size of electrode.
The symmetrical prismatic electrode must be centrally positioned in a vacuum chamber, however, and requires substrate holding devices because no more than one of the substrate support surfaces can be horizontal facing upwards. Many commercial sputtering systems, and particularly those used for integrated circuit production on ceramic wafers, are arranged to process the wafer substrates lying flat. A symmetrical prismatic electrode is not adapted for installation in such equipment.
In addition, the plasma region produced by such prismatic spool-shaped electrodes tends to be nonuniform, since the belt-like magnetic field bulges outward at its center region. This causes the plasma thickness to be greater at the center region than at the ends of the electrode body, thereby resulting in a non-uniform processing of the substrate surfaces.
A pending U.S. patent application, Ser. No. 06/461,022 now U.S. Pat. No. 4,581,118, owned by the assignee of the present application, proposes a magnetically enhanced substrate support electrode that is adapted for use in a chamber where the substrate lies flat and is intended to produce uniform plasma processing of an exposed surface of a substrate supported by the electrode. The electrode includes a rectangularly parallelepipedal body, the thickness of which is substantially less than its width and length, and two magnetic members constituting a first magnetic pole of one polarity disposed at one end of the body and a second magnetic pole of opposite polarity at the other end of the body.
One face of the electrode body is a substrate support surface, and each magnetic pole member projects beyond this face for the full width of the electrode body, so that a magnetic field extends longitudinally between the first and second poles for the full width of the electrode body adjacent to the substrate support surface. Preferably, the magnetic pole members project from both faces and from the side edges of the body to form a continuous peripheral flange at each end such that the magnetic field between the first pole and second pole surrounds the electrode body like a belt.
To improve uniformity of processing of the exposed surface of a substrate placed on the electrode, an auxiliary magnet means can be positioned in spaced relation to and facing the electrode support surface, the auxiliary magnet means having a third pole member positioned adjacent to the first pole member of the electrode and having the same polarity. A fourth pole member of the auxiliary magnet is positioned adjacent to the second pole member of the electrode. The strength and location of the auxiliary magnet are such that the resultant field adjacent to the substrate support surface is flattened and extends substantially parallel to the support surface.
The flattened field obtained by using the auxiliary magnet arrangement described above improves the uniformity of etching, for example, over most of the surface of a silicon wafer, but it has been found that there still remains a region, known as a "hot spot", where etching occurs at a significantly greater rate than over the rest of the substrate.
The "hot spot" region is located at the so-called leading edge of the support electrode, the leading edge being defined in relation to the direction of electron drift. In a magnetically enhanced plasma system in which the magnetic and electric fields cross each other substantially at right angles, electron drift takes place in an epicycloidal path that extends generally in a direction mutually orthogonal to the magnetic and electric fields. For a rectangular workpiece support electrode of the type described above, the electric field is perpendicular to the substrate support surface, and the magnetic field extends parallel to the support surface in the lengthwise direction of the electrode. Consequently, electrons drift generally from a first side edge to the opposite side edge of the electrode. The first side edge is the leading edge. Because of the magnetic and geometric structure of the electrode and the auxiliary magnet, there is some increased concentration of reactive ions in this leading edge "hot spot" region.